For that you first create a directory inside your res directory. By default we call this new directory raw, since it contains raw files like mp3. Right now dragging and dropping has some issue in Android Studio, so you'll have to manually go the folder where your project is and create copy paste the file to the raw directory.

Inside your code use the MediaPlayer method to access and call your mp3 file and play it Android Media Player

Hope this helps. Ask again if you have doubts